Trophic levels
Every organism belongs to a definite trophic level according to how it obtains its food. The links of a food chain are the trophic levels. Their number cannot be fixed, but chains usually have fewer than five links; the last link is a predator that eats flesh.
By how they obtain food, all living things fall into three main groups: autotrophs, heterotrophs and decomposers.
Autotrophs — producers
Organisms that can turn simple inorganic substances into organic compounds to supply their food — green plants, algae and some bacteria. By the energy source they use, they are photoautotrophs (green plants) or chemoautotrophs (some bacteria).
Heterotrophs — consumers
Animals that cannot make their own food and depend on food made by other organisms. Primary consumers are herbivores that feed on producers; secondary consumers are carnivores, or may be omnivores, that eat primary consumers; tertiary consumers are carnivores.
Decomposers
Saprophytic micro-organisms such as bacteria and fungi that get energy by breaking down the complex organic compounds in dead bodies and waste into simple compounds. That process is decomposition.
Figure to be added
Assignment 15.4
Observe the different ways the organisms around you get their food, and write down the feeding relationships between them.
Why decomposers close the loop
Without decomposers, dead leaves and bodies would pile up and the minerals locked in them would never return to the soil; plants would run out of nitrogen and phosphorus, and every chain would stop at its start. Decomposers feed on the dead of every level, so they do not sit at one place in the chain — they recycle what all the others leave behind.
Producers, consumers, decomposers
Autotrophs make food; heterotrophs eat it at primary, secondary and tertiary levels; decomposers break the dead down into simple compounds.